1. Field of the Invention
The present invention relates to a control device for a capacity-variable oil pump, and particularly relates to a capacity control on an oil pump provided in an engine using alcohol-containing fuel.
2. Description of Related Art
Conventionally, an engine is generally provided with an oil pump for lubricating a cylinder, a piston, a crank journal, and the like appropriately, and the oil pump is driven by a crankshaft via a chain or a gear. In view of this, in order to reduce a power loss (pump driving loss) of the engine due to the driving of the oil pump, it is suggested that a variable-capacity oil pump is used (see Japanese Patent Application Publication No. 2012-132356 (JP 2012-132356 A)).
In the meantime, so-called alcohol-containing fuel in which gasoline is mixed with alcohol may have been used in an automotive engine in recent years. Properties of such fuel change depending on a concentration of the alcohol, so a correction control to change an operating condition of the engine, such as an air-fuel ratio, is performed according to the concentration, for example. Further, it is also suggested that a control on the oil pump is corrected according to the alcohol concentration in the fuel.
As an example, in an alcohol engine described in Japanese Patent Application Publication No. 5-098921 (JP 5-098921 A), an amount of oil to be supplied to a trochoid inner peripheral surface of a rotary piston engine is corrected to be increased as an alcohol concentration in fuel is higher. That is, a specific heat of alcohol is ⅙ of that of gasoline, and a supply amount of the fuel increases as a concentration of the alcohol is high. As a result, the oil is washed away by the fuel on the trochoid inner peripheral surface, which results in that an oil film is easily discontinued.
However, in the latter conventional example, although the concentration of the alcohol contained in the fuel is considered, inconvenience caused due to mixing and accumulation of the alcohol in the oil is not considered at all. That is, alcohol has a lower volatility than that of gasoline. Therefore, as dilution of the oil by unburned fuel is promoted, the alcohol is accumulated in the oil. In a case where the engine is repeatedly operated without warming up due to short trip, the concentration of the alcohol mixed in the oil may increase rapidly.
An inventor of the present invention found the following fact. That is, when warming up of the engine is finished in a state where the concentration of the alcohol is high as described above and a temperature of the oil exceeds a boiling point of the alcohol in part of an oil supply system, the alcohol vaporizes at a stretch, so that a substantial flow rate of the oil decreases, which causes insufficient supply of the oil to lubrication portions.